1. Field of the Invention
This invention relates generally to magnesium-based alloys and more specifically to magnesium alloy compositions and methods of producing them that are for use in the automotive industry.
2. Background Art
There are various alloys developed for special applications including, for example, die casting of automotive components. Among these alloys magnesium-aluminium alloys can be designated as cost-effective and widely used for manufacture of automotive parts, e.g. AM50A alloy (where AM means aluminium and manganese are in the composition of the alloy) containing approx. 5 to 6 wt. % aluminium and manganese traces, and magnesium-aluminium-zinc alloys, e.g. AZ91D (where AZ means aluminium and zinc are in the composition of the alloy) containing approx. 9 wt. % aluminium and 1 wt. % zinc.
The disadvantage of these alloys is their low strength and poor creep resistance at elevated operating temperatures. As a result, the above mentioned magnesium alloys are less suitable for motor engines where some components such as transmission cases are exposed to temperatures up to 150° C. Poor creep resistance of these components can lead to a decrease in fastener clamp load in bolted joints and, hence, to oil leakage.
Known in the present state of art is a magnesium-based alloy (Inventors' certificate No. 442225 issued in Invention Bulletin 33, 1974) containing aluminium, zinc, manganese, and silicon as alloying components in the following amounts:    Aluminium—6–15 wt. %    Zinc—0.3–3.0 wt. %    Manganese—0.1–0.5 wt. %    Silicon—0.6–2.5 wt. %    Magnesium—balance.
The disadvantages of this alloy are its low ductility, high hot shortness, and insufficient strength of the alloy which keeps this alloy from automotive applications.
Known presently is another magnesium die cast alloy (“Magnesium alloys” in Collected works of Baikov Institute for Metallurgy edited by Nauka Publishing House, 1978, p.140–144) which comprises aluminium, zinc, manganese, and silicon as alloying components in the following amounts:    Aluminium—3.5–5.0 wt. %    Zinc—under 0.12 wt. %    Manganese—0.20–0.50 wt. %    Silicon—0.5–1.5 wt. %    Copper—under 0.06            Nickel—0.03 wt. %        
The drawback of this alloy is that the quantitative composition of the alloy selected provides poor mechanical properties, in particular, the alloy having a small solidification range is characterised with advanced susceptibility to cracking in case of hindered contraction and bad castability.
A well-known German standard EN 1753-1997 is taken as the closest prior art by its qualitative and quantitative composition and discloses the methods of manufacture of EN MB MgAl2Si and EN MB MgAl4Si alloys. The qualitative analysis of the alloys is the following, in wt. %:    EN MB MgAl2Si:    Al—1.9–2.5    Mn—min 0.2    Zn—0.15–0.25    Si—0.7–1.2EN MB MgAl4Si (AS41):    Al—3.7–4.8    Mn—0.35–0.6    Zn—max 0.10    Si—0.6–1.4
The alloys of the above quantitative and qualitative compositions demonstrate better mechanical properties. However, at 150–250° C. these alloys have high creep that keeps these alloys from machine-building applications. Presently known is the method (PCT Patent No. 94/09168) for making a magnesium-based alloy that provides for alloying components in a molten state being introduced into molten magnesium. Primary magnesium and alloying components are therefor heated and melted in separate crucibles.
A disadvantage of this method is the need to pre-melt manganese and other alloying elements (at the melting temperature of 1250° C.) that complicates alloy production and process instrumentation.
There are some other methods known (B. I. Bondarev “Melting and Casting of Wrought Magnesium Alloys” edited by Metallurgy Publishing House, Moscow, Russia 1973, pp 119–122) to introduce alloying components using a master alloy, e.g. a magnesium-manganese master alloy (at the alloying temperature of 740–760° C.).
This method is disadvantageous because the alloying temperature should be kept high. This leads to extremely high electric power consumption for metal heating and significant melting loss.
Also known is another method of producing a magnesium-aluminium-zinc-manganese alloy (I. P. Vyatkin, V. A. Kechin, S. V. Mushkov in “Primary magnesium refining and melting” edited by Metallurgy Publishing House, Moscow, Russia 1974, pp. 54–56, pp. 82–93) which is taken as an analogue-prototype. This method stipulates various ways for feeding molten magnesium and alloying components such as aluminium, zinc, and manganese. One of these approaches includes simultaneous charging of solid aluminium and zinc into a crucible, then heating above 100° C., pouring in molten primary magnesium and again heating up to 700–710° C. and introducing titanium-containing fusion cake together and manganese metal under continuous agitation.
The main shortcoming of the method is in considerable loss of alloying components, resulting in lower recovery of alloying components in magnesium and preventing from producing alloys with specified mechanical properties. Furthermore, this increases the cost of the alloy.